This invention concerns adjustment of a radar detection threshold to minimize parasite echoes known under the general term of clutter, while minimizing desensitization of the radar to avoid the loss of interesting targets. It makes use of the technique known as CFAR (Constant False Alarm Rate). It is particularly concerned with adjustment of the target extraction threshold from a panoramic marine patrol radar.
In radar technique, an observable target is a target that creates an echo with an amplitude exceeding that of the surrounding noise. In order to distinguish it from the noise, a detection threshold has to be fixed lower than the amplitude of its echo and higher than the amplitude of the surrounding noise. Given the random nature of the noise, this threshold is defined as a function of an acceptable false alarm rate that we attempt to make constant, despite the various probability density functions that may represent clutter, and particularly sea clutter.
In order to use a radar reception signal, the signal is sampled over a period of time and samples are grouped by category each concerning a single cell corresponding to a given radar pointing direction and a given target distance in this pointing direction. A known manner of adjusting the detection threshold in a distance-direction cell is to evaluate the average value of the signal received on distance-direction cells surrounding the distance-direction cell considered for a given pointing direction and lower and higher distances, and to use two to four times this average value as a detection threshold.
This method of calculating the detection threshold has the disadvantage that it requires many calculations for a panoramic radar with a high number of distance-direction cells. Also in the case of a marine patrol radar, powerful coastal echoes increase the value of the detection threshold thus making the radar less sensitive in coastal areas.
In order to reduce the number of calculations, we could envisage breaking down the area monitored by the radar into sectors each containing several distance-direction cells within which clutter remains uniform and using a detection threshold S for each sector representing a set value of the false alarm probability Pfa. This could be done by defining a clutter histogram for each sector that, assuming that signals returned by interesting targets are the exception, becomes coincident with the histogram relating possible values, x, of signal moduli received by the radar for the sector considered with their appearance frequencies, y, and determining the threshold S using the relation defining the false alarm rate Pfa that is the limit of the following integral when the total number N of events contained in the histogram tends towards infinity: ##EQU2##
Unfortunately if it is required to limit the amount of calculation, the number of events contained in the histogram has to be limited such that the behavior of the histogram is not sufficiently accurate for high values of received signal moduli to be able to deduce a significant threshold value using the following relation: ##EQU3##
If the probability density function f(x) describing clutter is known, this difficulty can be overcome by replacing the histogram y(x)in the relation defining the false alarm probability Pfa by the probability density function f(x). We obtain: ##EQU4##
This is a known averaging method, known as "cell average" that is used when the main source of clutter is thermal noise and the probability density function is of the Rayleigh type ##EQU5## where m.sub.2 is the root mean square. In this method, the average is calculated on each sector and the root mean square of the clutter is deduced using a ratio of 1.25, such that the applicable Rayleigh function can be defined, and the threshold S is then obtained from tables of values containing results obtained from using relation (1) for the various possible values of the set false alarm probability and the root mean square.
However this cell averaging method can not be used in the case of a marine patrol radar subject to non-uniform clutter that depends on sea conditions and the direction of the prevailing wind, and for which the probability density function satisfies a log-normal type function at low radar distances particularly when the sea is rough, and a Rayleigh type function beyond a given "transition" distance. Moreover, it does not solve the problem caused by coastal echoes since they are too numerous in some sectors to remain an exception and have a non-negligible influence on the root mean square of received signal moduli assigned to clutter.